The present invention relates generally to power conversion systems and more particularly to a system for controlling the torque of an AC motor load, not only in a substantially constant slip mode, but in a voltage limited mode with reduced flux beyond a predetermined speed.
In the past DC (direct current) motors have been used for operation over wide speed ranges. More recently, AC motors have been finding greater application in variable speed drive applications. This is due in a large part to the inherent ruggedness of an AC induction motor, for example, that which results in a reduced maintenance problem due to the lack of brushes which makes AC motors particularly desirable for certain applications. There are however, certain problems associated with the use of AC motors, particularly when the motor is supplied by power from a variable frequency inverter (DC to AC) which in turn is fed DC power from a converter (AC to DC).
Where, for example, the converter/inverter circuits are implemented by means of phase controlled thyristors connected in bridge circuit configurations, one must consider the commutation delay existing between the time current transfer is effected from a previously conducting thyristor to the newly fired thyristor. Normally this overlap is from 10 to 30 electrical degrees. In supplying an AC motor, such as an induction motor at high motor speed, it may take as long as 120 electrical degrees between the firing of a new thyristor and the transfer of the current to the motor phase to which the thyristor is connected. As such, the use of an open loop system in which thyristors are fired as a result of a preestablished schedule is not particularly applicable to the desired type of operation and instead a closed loop system in which the thyristors, for example, are fired as a function of the existing angle .theta. between the air gap flux and the motor current (commonly referred to as the air gap power factor) represents a more desirable mode of operation.
In the above referenced related U.S. Pat. No. 4,230,797, which is specifically incorporated herein by reference, there is disclosed a controlled current inverter system for supplying an AC load such as an induction motor with an AC current of variable magnitude and variable frequency. The system described therein employs a variable DC power source which is connected to a variable frequency converter preferably by way of a DC link including an inductor. Means are included to develop signals representing the instantaneous electrical torque T of the AC motor and the instantaneous air gap power factor. The electrical torque signal and the angle signal are utilized to control the DC current in the link and the firing angle of the inverter with respect to the motor flux such that the angle .theta. is controlled and maintained substantially constant over its prescribed operating range.